The lack of adequate quantities of fresh water poses a significant global challenge given that about 97% of the Earth's water is seawater. Seawater is non-potable and cannot be used for agricultural irrigation. As such, improved methods and systems for wastewater treatment and/or desalinating water may be critical for producing fresh water, especially in areas where seawater is abundant, but fresh water is not.
A variety of technologies have been employed to produce fresh water from wastewater. One such technology is forward osmosis, which is the movement of water across a semi-permeable membrane in order to induce flow from an area of high-water potential to an area of low-water potential. Particularly, the driving force for forward osmosis is a concentrated solution (i.e., a draw solution) located on a permeate side of the semi-permeable membrane. The permeate is typically highly soluble in water, has a low molecular weight, and is easily and inexpensively separable from the draw solution to leave potable water. Treatment of wastewater via forward osmosis, however, does not yield fresh water, but instead produces water still containing organic contaminants and thus the water is in need of further treatment.
A second such technology is a microbial fuel cell (MFC) in which electricity may be harvested directly during microbial metabolism of organic matter. Specifically, in a MFC, organic matter (e.g., organic contaminants in wastewater) is metabolized by microbes in an anode chamber thus transferring electrons to the anode and liberating protons into the aqueous phase such that the electrons flow through a wire from the anode to a cathode to produce an electrical current. At the cathode, the electrons are accepted by a terminal electron acceptor (e.g., oxygen). Additionally, ion transport between the anode and cathode is needed to maintain proper change balance in the microbial fuel cell and to facilitate the generation of electricity. A microbial fuel cell, while removing organic contaminants from wastewater, does not remove other contaminants (e.g., non-organic) and thus further treatment is required to obtain fresh water.
Further, MFCs can be modified so as to be able to desalinate water concurrently with the treatment of organic wastes and the production of electricity. Specifically, MFCs can be modified to include a saline solution chamber positioned between the anode and the cathode; where the saline solution chamber contains an aqueous solution including anions and cations. When electricity is generated in such a modified MFC, the cations in the aqueous solution move through a cation exchange membrane (CEM) to or toward the cathode, while anions in the aqueous solution move through an anion exchange membrane (AEM) to or toward the anode. Accordingly, ion transport maintains a proper charge balance between the anode and cathode while concomitantly separating the cations and anions from the aqueous solution in the saline solution chamber, thereby desalinating the aqueous solution in the saline solution chamber. These modified MFCs are commonly referred to as microbial desalination cells (MDCs).